Human monocytes are activated by endotoxin and other inflammatory microbial products to produce harmone-like factors such as interleukin l (IL 1). IL 1 enhances lumphocytes' mitogenic responses to lectins and thus appears to be a nonspecific lymphocyte activating signal that acts jointly with the antigen or polyclonal stimulant to induce T cell activation. IL 1 from human monocytes has been purified 20,000 fold by the sequential use of ultrogel AcA54 column, affinity chromatography and isoelectrofocusing. A human monocyte leukemia cell line (THP 1) was found to produce IL 1 upon stimulation and can potentially be used to produce more IL 1 for biochemical purification and molecular cloning of the IL 1 gene.